It is state of the art to use diffractive optically variable image devices (DOVIDs) like holograms for anti-counterfeiting of banknotes or credit cards. Further magnetic codes or fluorescent dyes are often used to prove the originality of items. Unfortunately, counterfeiters have already produced forged high-quality versions of devices using all those techniques. Especially DOVIDs possess only a low level of security, because non-experts generally do not know how the holographic image should look. Therefore, there is a need for novel security devices that are more difficult to counterfeit.
Optically variable inks (OVIs), as disclosed in U.S. Pat. No. 4,705,356, provide a higher level of security, as it is easier for non-experts to observe a color change than a complex image. Although OVIs are also difficult to manufacture, and therefore seem to be secure, their effect can be closely mimicked with color-shifting inks used for decorative purposes that are commercially available from several companies such as JDS Uniphase Corp., San Jose, Calif. This decreases the value of OVIs as anti-counterfeiting tool.
In U.S. Pat. No. 4,484,797, color filters with zero-order microstructures are described for use as authenticating devices. Illuminated even with non-polarized, polychromatic light, such devices show unique color effects upon rotation and therefore can be clearly identified. However, due to the fact that the filters are based on the resonant reflection of a leaky waveguide, they possess narrow reflection peaks and thus produce weak color effects. Moreover, the possibilities for varying the color effect are limited.
WO-03/059643 also describes zero-order diffractive gratings for use in security elements. The elements have the same drawbacks as the filters in U.S. Pat. No. 4,484,797.
Zero-order diffractive filters illuminated by unpolarized polychromatic visible light are capable of separating zero-diffraction-order output light from higher-diffraction-order output light. Two examples of such filters 101 are shown in FIG. 1. The filters 101 consist, for instance, of parallel grating lines of a material 132 with relatively high index of refraction surrounded by materials 131, 133 with lower indices of refraction. The high-index layer 132 acts as a leaky waveguide. The materials 131, 133 above and below the waveguide 132 can have equal or different indices of refraction and one of them can even be air. The grating lines are parallel to the y direction. Λ denotes the period of the grating, p the width of the grating grooves, c the thickness of the high-index-of-refraction coating (black), t the depth of the grating structure, Θ the incidence angle and φ the rotation angle. φ=0° is defined as incident direction perpendicular to the grating lines (i.e., a straight line lying in the (xz) plane).
These filters 131 possess characteristic reflection and transmission spectra depending on the viewing angle and the orientation of the grating lines with respect to the observer (see M. T. Gale, “Zero-Order Grating Microstructures”, in R. L. van Renesse, Optical Document Security, 2nd Ed., pp. 267-287). Other parameters influencing the color effect are, for example, the period Λ, the thickness c of the high-index layer 132, the grating depth t, the fill factor f=p/Λ and the shape of the microstructure (rectangular, sinusoidal, or more complex). In reflection, the filters 101 show a color which varies with the viewing angle. As the filters reflect light in the zeroth order, this viewing angle is equal to the incidence angle Θ. As long as the materials 131-133 used show no absorption, the transmission spectra are the complement of those in reflection. A characteristic feature of such filters 101 is a color change upon rotation by 90°. However, the intensity of light reflected by such filters 101 is low.